1. Field of the Invention
This invention relates to a scanning tunneling microscope which can be used to observe the surface of a sample in the unit of atom size.
2. Description of the Related Art
Recently, a scanning tunneling microscope (STM) which can be used to observe the surface of a sample in the unit of atom size has been developed.
It is generally well known in the art that when a metal probe having a sharp tip end is set as close as approx. 1 nm to the surface of an electrically conductive sample and a preset voltage is applied between the probe and the sample, a tunnel current flows between the probe and the sample. The tunnel current is largely dependent on a distance between the probe and the sample. The STM utilizes the property of the tunnel current to observe the surface of the sample. That is, when the probe is mounted on the actuator which can be moved in a 3-dimensional direction and it is scanned while the tunnel current is kept constant, the probe will move along the irregularity of the surface of the sample with a preset distance kept therebetween. Thus, variation of the surface state of the sample can be observed in the unit of atom size as an image by outputting the position of the probe as a 3-dimensional image.
In general, a servo circuit is used to adjust the distance between the probe and the sample in the STM. The servo circuit detects a tunnel current flowing between the probe and the sample and controls the driving operation of the actuator to keep the distance between the probe and the sample at a constant value based on the detected tunnel current.
In the conventional STM, adjustment of the distance between the probe and the sample is controlled only by use of the above servo circuit. Therefore, when the surface condition of the probe is bad, desired control cannot be effected. That is, when the sample has a slanted surface, undulated surface or a surface having holes formed therein and if the servo output is displayed on a CRT as an output indicating the surface condition of the sample based on the detected tunnel current, the CRT image plane cannot be effectively used and the STM image cannot be displayed within the image plane. In this case, the distance between the probe and the sample cannot be controlled by the servo system so that the STM image obtained can be displayed only with a low resolution in a vertical direction and the dynamic range thereof may be deviated from the central position.
Further, as described before, the STM is a microscope having a super high resolution and can be used to observe the surface configuration and surface properties of the sample in the unit of atom size. Therefore, in a case where a desired portion of the sample is observed, it is necessary to first observe a wide range (several .mu.m) previously set to include the desired portion and then observe the desired portion.
Further, when an STM image (3-dimensional image) of the wide scanning range is observed, it is sometimes required to enlarge part of the image for more specific observation.
In order to meet the above requirements, in the prior art, an image obtained by wide range scanning with a preset resolution is displayed, then the scanning range is changed to a narrow scanning range and the probe is upwardly moved away from the sample surface by such a distance that the tunnel current cannot flow. Then, the scanning center is set on the position at which a portion to be enlarged lies by an X-Y rough moving mechanism using a pulse motor and the like and the lifted probe is set closer to the sample surface so as to be set within a tunnel current region, and then the scanning operation is effected again to display an image.
However, in the above method, the precision of the rough moving mechanism for moving the sample influences the reliability of the STM image. Actually, the precision of the rough moving mechanism is extremely lower than the resolution of the STM. According to the method in which the probe is separated from the sample after the wide range scanning operation is effected, and then moved by use of the rough moving mechanism and set closer to the sample again to display an STM image, the desired position cannot always be correctly set because of the low resolution and precision of the rough moving mechanism. Therefore, an enlarged image at exactly the desired position cannot always be obtained.